This invention relates to a new material. More specifically, this invention relates to a new multilayer crystalline structure which is coherent perpendicular to the layers and in which the layers are ultrathin alternating layers of crystalline materials which are not semiconductor compounds. The invention also relates to a method of making this new multilayer crystalline material.
Crystalline, coherent, multilayered structures having very thin component layers that differ in composition but which are lattice-matched, generally known as superlattices, form the basis for a number of different semiconductor devices. The semiconductor superlattices have been used to prepare solid state lasers and as optical modulators, optical switches, waveguides and couplers.
It has been universally accepted that these structures can only be produced without defects if the constituents have the same crystal structure and closely matching lattice parameters. The defects or dislocations result when the lattice parameters differ too greatly, imposing an elastic strain upon the lattice. In addition, if the components are not properly matched, diffusion will occur between layers which will destroy the superlattice structure.
Heretofore, these superlattice structures have only been prepared of alternating layers of lattice-matched compounds which have generally been semiconductors. The constitutents are usually binary compounds of one of the group IIIA elements Al, Ga and In and one of the Group V elements P, As and Sb. The superlattices are then formed by layering, for instance GaAs and AlAs. These have the same crystal structures and only a small variation in lattice spacing. These semiconductor superlattice structures may be formed by a number of different techniques. For example, a superlattice structure is produced by epitaxially growing a semiconductor material such as GaAs, which is periodically doped so as to produce alternating ultrathin layers having different conductivity types. More recent methods for the preparation of semiconductor superlattices include vapor phase and liquid phase epitaxy. Molecular beam epitaxy has become particularly successful because it provides the ultra high vacuum and high purity conditions believed necessary to produce high quality superlattice material. However, none of these methods have been successful in producing multilayered crystals from materials which are not lattice-matched.